In order to prevent illegal use of articles, for example, various cards for adjustment, and certificates, etc., techniques for determining whether or not they are authentic are necessary.
Techniques of applying a special ink on a surface of an article have been known as the above technique. For example, predetermined characters or figures may be printed on an article by using an ink which is fluorescent under ultraviolet light is used as the above ink. When ultraviolet light is irradiated on the article, the characters or patterns become visible on the article, so that the authenticity of the article can be determined. For example, an ink having particles of a magnetic material or magnetized particles mixed therewith may be applied on an article, and the authenticity of the article can be determined by magnetic sensors.
Techniques of using a hologram for determining the authenticity of an article have been known. Techniques of using optical characteristics of cholesteric liquid crystal have been known as techniques for prevention of falsification of securities by copying, as disclosed in Patent Publication 1.
The Patent Publication 1 is Japanese Unexamined Patent Application Publication No. Hei 4-144796.